Spray-drying is the standard method for manufacturing laundry detergent base powder. Typically, detergent ingredients are mixed together to form an aqueous detergent slurry in a mixer, such as a crutcher mixer. This slurry is then transferred in and along a pipe through at least one pump to a spray nozzle, and the slurry is sprayed into a spray-drying tower, and spray-dried to form a spray-dried powder.
It is beneficial to have certain detergent ingredients in the same detergent particle. This avoids any problem of chemical variable and performance differences of each dose, due to product segregation which may occur when the laundry detergent product consists of numerous chemically distinct separate particles. However, certain detergent ingredients may be incompatible with other detergent ingredients, especially when they undergo a mixing step, especially an excessively prolonged mixing residence time, such as the mixing step that typically occurs in a crutcher mixer. These incompatible detergent ingredients can be introduced to the aqueous detergent slurry at a later stage in the spray-drying process, for example by injection into the pipe after the mixer but before the spray nozzle, or even after the pump and before the spray nozzle. This minimizes that amount of mixing and contact time between the incompatible detergent ingredients and the other detergent ingredients present in the aqueous detergent slurry.
However, even when these incompatible detergent ingredients are introduced at a later stage in the spray-drying process, there remains a need to further ensure that substantially no mixing occurs between these incompatible detergent ingredients and the other detergent ingredients already present in the aqueous detergent slurry until as late as possible in the spray-drying process, such as only at the spray nozzle.
The Inventors have overcome this problem by introducing these incompatible detergent ingredients to the aqueous detergent slurry at a later stage in the spray-drying process in such a manner so as to limit the extent of mixing that may occur. The Inventors carefully ensure that the detergent ingredient is contacted to an aqueous detergent slurry that is in laminar flow, and then carefully control the flow rates of both the aqueous detergent slurry and the detergent ingredient to ensure that the resultant mixture is also in laminar flow. In this manner, turbulence of the fluid in the pipe is avoided and the degree of axial mixing that may occur is substantially minimized.